(A) Field of the Invention
The present invention relates to an apparatus for measuring spectrographic images, and more particularly, to an apparatus for measuring spectrum and image with high spatial resolution and spectral resolution.
(B) Description of the Related Art
Generally speaking, an optical image technique can only measure spatial information of an object, i.e., the image of the object, but cannot acquire spectral information. To acquire both spatial and spectral information, the optical image technique needs extra spectral measuring technique. In other words, the spectral information can only be obtained when an additional light-splitting device is incorporated into an optical imaging system.
Conventional spectrographic images measuring technique can be classified as a point scanning and a global imaging. The point scanning take a relatively long measuring time for moving the probe or the object in two-dimensional manner and the measured data needs to be combined point by point, so the obtained image tends to distort. On the contrary, the wavelength resolution of the global imaging is limited for using light-splitting devices such as the optical filter. Consequently, the above-mentioned two methods are not suitable for measuring an object with a larger area rapidly and at high spatial/spectral resolution. Particularly, to measure an object with large area rapidly at high spatial/spectral resolution can be realized only by line scanning. The line scanning method allows acquiring the spectrographic images of the object just by moving in a single direction. Therefore, it has advantage of rapid measuring speed and the image mapping is simpler without distortion. In addition, the line scanning possesses a higher spectral resolution due to the dispersing device.
Conventional spectral image measuring apparatus possesses a line-shaped field of view (FOV), and the light spot of the object in the field of view after passing lens, reflecting mirror and dispersion device causes great aberration such as spherical aberration, coma aberration, and chromatic aberration. Theses aberration results in severe expansion and distortion of imaging light spots on 2D sensor, and adjacent imaging light spots is not distinguishable due to overlapping. Consequently, neither the spatial nor spectral resolution cannot be improved. Hence, it needs a new design for improving the resolution to develop the spectral machine vision.
FIG. 1 is an apparatus 10 for measuring spectrographic images according to the prior art. The apparatus 10 uses an optical collector 30 to guide optical energy 14 from points on the Y axis in the field of view of an object 12 to a spherical lens 18 after penetrating through a optical slit 16. The optical energy 14 is collimated by the spherical lens 18, and then enters into a diffraction grating 20 to disperse into rays 22 with different wavelengths and take-off angles. The ray 22 is focused on a charge-coupled device (CCD) 26 by a focusing lens 24 to simultaneously pick up the spatial and spectral information of the object 12. The opening of the slit 16 in FIG. 1 is parallel to Y-axis by the long side, and to X-axis by the short side.
FIG. 2(a) is a schematic diagram of the collector 30 according to the prior art. The collector 30 uses a multi-track fiber head 40 including several fibers 42 for close measurement of the object 12. The multi-track fiber 40 is inserted into an F-number matcher 43, and the optical energy in the three fibers 42 can present three light spot 46 as shown in FIG. 2 (b) at the optical slit 16 by the convergence of the reflecting mirror 44 and the concave reflecting mirror 45. However, the size and spatial resolution of the analyzable field of view on the object 12 depend on the arrangement, the diameter and quantity of the fiber 42 of the multi-track fiber 40. Consequently, available channels are limited, and the channel of the collector 30 in FIG. 2 is only 3. In addition, the optical energy 14 can be collected from the object 12 only by closing the multi-track fiber 40 to the object 12, which results in difficulty in measuring. Therefore, such a design is mainly used to measure the spectrographic images in an experiment at a lower resolution requirement.
FIG. 3 shows the operation of the optical collector 30 using an imaging lens 50 according to another embodiment of the prior art. The imaging lens 50 collects the optical energy 14 to the optical slit 16, and guides the optical energy 14 to a grating 56 via a spherical lens 54. The width of the optical slit 16 and the size of the CCD 26 determine the size of analyzable FOV of the object 12. However, off-axis light beams of the object 12 enter into the optical slit 16 via the imaging lens 50, and the principle ray and optical axis 58 form an included angle θ1, i.e., the principle ray is not parallel to the optical axis 58. As a result, the off-axis light beam causes a great de-collimation after passing the spherical lens 54, which cannot meet the requirement that the light beam enters into the grating 56 at a collimated manner, and the spectrum resolution present on CCD 26 is reduces. In addition, such a de-collimation will also cause extra aberration, which further reduces the spectral resolution on the CCD 26. Therefore, the channels available to measure cannot be increased due to the limitation of the spectral/spatial resolution. Hence, such a design can only be used in the comparison with low resolution, and cannot generate the spectrographic images with real high spatial/spectral resolution.